I have to say I have used marvel air tool oil; for 20 years and it is really good stuff. It does not gum up since it is designed as a oil that actually prevents and reduces varnish buildup. Hence the use in air tools. I oil my engines once a year and put them in a baggy(ziplock). I have engines that are now going on 42 years old from my control line days, and they all run fine after storing them with the marvel air tool oil. NOTE air tool oil cannot be used in YS engines and other pump engines as is softens the pump diaphragm.